If you already have a 3.x version on your k2/DX kindle that's probably not what you want.

That said you can of course also update your k3 to 3.3 and create a new "restore" package with my files, at least if you still have the rootfs.img created during the initial upgrade to 3.x.
That would also install the 3.3 version on your k2/dx kindle. But it would "restore" the device - including a factory reset - to 3.3, not update it.

So the only concern when using the bin-file generated by your scripts is that it will factory-reset the k2/DX?

And just to double-check: to make your 3.3 script create the correct bin-file do I need to use the rootfs.img which was created by the back up script while DX was on 2.5.8 or do I need to generate a new one on my DX (which is now on 3.2.1) if I want to migrate from 3.2.1 to 3.3?